The invention relates generally to measuring the extinction ratio (Er) of an optical transmitter. More specifically, the invention relates to systems and methods that use interferometry to derive a relationship between interferogram wing-to-peak ratios (Iwp) and measured Er for an optical transmitter and may use the relationship after deployment to determine an Er by measuring an Iwp.
The parameter that best describes the overall health of a communication system is Bit Error Ratio (BER). Most digital communications systems are capable of error-free communication if transmitter powers are kept high enough and fiber attenuation is kept low enough. To obviate costly optical regenerators, it is desirable to have the longest span possible between a transmitter and receiver.
Lengthening the transmission span too far will eventually degrade the system BER, as signal levels drop and noise becomes dominant over the signal at the receiver. However, transmitter extinction ratio (Er) will also impact the allowable length of a transmission system. Historically, it has been difficult to achieve accurate and repeatable Er measurements.
Er is used to describe optimal biasing conditions of optical transmitters and how efficiently the available laser transmitter power is converted to modulation power. For example, a transmitter for long-haul transmission may comprise a distributed-feedback laser and an external Mach-Zehnder modulator. FIG. 1 shows a modulator as an integrated optical waveguide on a material that exhibits an electro-optic effect, where the material's index of refraction can be changed by applying a voltage. The purpose of the modulator is to convert an electrical data signal of logic level 1's and 0's, shown as the voltage V(t) applied relative to ground (denoted as 0V), to an optical signal at its output I(t) that is “on” for a logic level 1 and nearly “off” for a logic level 0. Due to the structure of the modulator and the electro-optic effect, the output power of the modulator will vary as
                                          I            ⁡                          (              t              )                                =                                                    I                0                            2                        ⁢                          (                              1                +                                  cos                  ⁡                                      (                                                                  π                        ⁢                                                                              V                            ⁡                                                          (                              t                              )                                                                                                            V                            π                                                                                              +                      ϕ                                        )                                                              )                                      ,                            (        1        )            
with the applied voltage V(t) as shown in FIG. 2. This relation is called the modulation transfer function, and I0 is the maximum power at the modulator output. The switching voltage Vπ is the difference in voltage required to switch the modulator from a full “on” state, where the output power is maximum, to a full “off” state, where the output power is minimum. The phase shift φ is nominally zero, but is included in (1) to account for slight shifts of the modulation transfer function due to temperature variation and modulator aging. The applied voltage V(t) can be divided into two partsV(t)=Vbias+VAC(t),  (2)
where Vbias is a DC bias voltage that is constant in time and VAC(t) is an ac drive voltage that varies in time. Vbias is applied to optimally operate the modulator and is usually set to position the modulator on the modulation transfer function so that the output power is one-half of its maximum when VAC(t)=0. An ac drive voltage VAC(t) with a magnitude of Vπ is then applied to switch the modulator from maximum transmission to minimum transmission, in accordance with the data signal (i.e. at the same rate and with the same pattern of logic level 1's and 0's). Vbias can be set to operate (i.e. bias) the modulator on the positive slope Vbias+ of the transfer function (i.e. where the modulator transmission increases with increasing applied voltage). Or Vbias can be set to operate the modulator on the negative slope Vbias− of the transfer function (i.e. where the modulator transmission increases with decreasing applied voltage).
Er is the ratio of the power used to transmit a logic level 1 to the power used to transmit a logic level 0 and may be defined as a linear ratio in decibels or as a percentage. In dB,
                              extinction          ⁢                                          ⁢          ratio                =                  10          ⁢                                          ⁢                      log            10                    ⁢                                                    logic                ⁢                                                                  ⁢                1                ⁢                                                                  ⁢                power                ⁢                                                                  ⁢                level                                            logic                ⁢                                                                  ⁢                0                ⁢                                                                  ⁢                power                ⁢                                                                  ⁢                level                                      .                                              (        3        )            
FIG. 3 shows an eye-diagram that is commonly used to illustrate Er. Transmitter Er is typically obtained from an eye-diagram.
Good BER performance is achieved when there is significant separation between the power level used to transmit a logical 1 and the power level used to transmit a logical 0. The difference between the two power levels describes the modulation power of the transmitted signal. The larger the modulation power, the easier it will be for a system receiver to accurately determine what signal level is present. Er values in the range of 10 to 20 (10 to 13 dB) are common for high-speed, externally modulated lasers used in optical transmitters.
As optical transmitters age, the switching voltage Vπ of the modulator may increase and/or the optimal bias voltage may increase or decrease compared to the original. If the modulator bias voltage and/or the AC drive are no longer of the correct value, then the optical power in the logic level 1's is less than the maximum output from the modulator and the optical power in the logic level 0's is greater than the minimum output from the modulator, and therefore the modulated signal's output Er degrades. Since Er is an important performance monitor parameter, other measurement methods besides using a fast digital oscilloscope are desired. What is desired is a system and method that measures the Er of an optical transmitter using interferometry.